In previous studies I found that there exists a mechanism whereby Chinese hamster ovary cells that survive a toxic dose of the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) are permanently more resistant to the killing effects of MNNG. This phenomenon of acquired resistance was found also in (human carcinoma-derived) HeLa cells that are MNNG-sensitive, i.e. display the Mer-phenotype. Resistance was not due to decreased MNNG uptake or binding to DNA or to higher overall DNA repair of methylated bases, in particular, resistant variants, like their sensitive parent, were deficient in repair of 06-methylguanine (06-MG). Therefore the two characteristics of the Mer-phenotype (MNNG-sensitivity and lack of 06-MG repair) that have always been linked, can be uncoupled. I propose to define the process by which increased resistance occurs and to investigate the cause for resistance systematically at a cellular, biochemical and molecular level using these sensitive and resistant variants. At the cellular level a genetic analysis will be attempted by producing cell hybrids between sensitive and resistant lines and determining their response to MNNG. To investigate the mechanism of increased resistance at the biochemical and molecular level it will be examined, if sensitive and resistant variants differ in an intracellular signal for DNA damage response or in inhibition and recovery of RNA synthesis, and it will be tested if resistant variants show preferential repair of alkylation damage in transcriptionally active genes. The results will increase our understanding of how alkylating agents cause cell killing and what tolerance mechanism are available to mammalian cells to overcome the toxic effects of these agents. Identification of this mechanism should also clarify some aspects of regulation of alkylation damage repair in mammalian cells, i.e., why this function is normally expressed together with the 06-MG repair protein.